According to Professors Cleveland M. Jones and Hernani A. F. Chaves, the assessment of a play (such as the pre-salt area) consists of various steps, such as establishing the order of discovery of known accumulations (Discovery Sequence), determining the size distribution for those discoveries (Field Size Distribution – FSD), estimating the number of possible accumulations of economic interest, quantifying the geological uncertainties and conducting simulations of the individual accumulation assessments.

Figure 1. Example of the probabilistic assessment result for an individual field

Probabilistic results are first produced for all individual accumulations considered in the area of study, including existing discoveries and postulated accumulations (Figure 1). All individual assessments are then combined, using Monte Carlo simulations, in order to build the overall assessment parameters (Figure 2).

Figure 2. Example of how assessment elements are included in the overall assessment

Since the total YTF oil potential depends on the number and size of fields, which take on random sampled values in the assessment methodology, that potential cannot be estimated by directly multiplying individual values – a stochastic simulation of the product is required. The software modelling tool employed makes intensive use of Monte Carlo simulations, which implies heavy computer use.

The results of the 2010 assessment indicated that in the pre-salt region there were a significant number of various size fields yet to be discovered, for a total YTF recoverable oil potential of between 114 billion (P90) and 288 billion barrels (P10).

Possibly because these numbers were far larger than even enthusiastic government officials were mentioning at the time, the results of the assessment were given relatively little attention by much of the academic and industry community. The results may have been considered rather unlikely, despite the robust methodology utilised, and the study was covered mostly by the international news media.

Since 2010, several years of intensive exploratory efforts followed, many new wells were drilled (some dry or uneconomical), new discoveries were made, and several prospects and fields were re-evaluated. Original estimates for some known accumulations were significantly reduced, and others significantly increased, as well as many new prospects identified.

Discovered accumulations of 30 to 50 billion barrels also came to be accepted as realistic, despite few details regarding how such volumes were calculated. Obtaining specific information required for inputting data on individual accumulations and prospects identified in the region, used in the assessment, remains one of the most difficult challenges of the assessment work. Such information had to be mined among company reports, public disclosures and media reports, since it is not routinely furnished by operators.

Based on the accumulated new information available, in 2015 we conducted a re-assessment of our 2010 study, utilising essentially the same methodology and some refinements.

The results of the 2015 assessment suggest that despite the unprecedented numbers of the 2010 assessment, those results were not at all far-fetched, and now there is even stronger evidence for validating the robust methodology employed.

Figure 3. Result screenshot for the number of YTF accumulations

The latest estimate is that there are still many more significant size fields (above a minimum of 20 million barrels) remaining to be discovered in the Brazilian pre-salt polygon. The number of such accumulations ranges from 56 (P90) to 83 P10) (Figure 3). The prior assessment in 2010 suggested that the number ranged from 43 (P90) to 83 (P10), so despite a significant amount of additional exploratory information considered since then, the results are within the original estimated range, and the current results represent a narrowing of the probabilistic interval, as expected.

Figure 4. Result screenshot for the size of the YTF accumulations

In the current assessment, the estimate is that for those fields, the individual size of the accumulations yet to be discovered will range between 240 million (P90) and 5.9 billion barrels (P10) (Figure 4). The prior assessment in 2010 suggested that those sizes would range from 166 million (P90) to 8 billion barrels (P10), so once again, as expected, the results are within the original estimated range, and the current results represent a narrowing of the probabilistic interval.

Figure 5. Result screenshot for the total recoverable resources in the area of study

As to total recoverable resources, considering only accumulations greater than a minimum field size of 20 million barrels, the 2015 assessment indicates that total to be between 176 billion (P90) and 273 billion barrels (P10) (Figure 5). That result also represents a narrowing of the 2010 assessment results, since at that time the total recoverable resources in the pre-salt region were estimated to be between 115 (P90) and 288 billion barrels (P10).